Question about Bosch AquaStar 125FXKIT-NG 125FX Natural Gas Tankless Water Heater w/ Hood

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Tankless water heater pilot light will not stay on

I have a Bosch Aquastar AQ 125 BNG tankless hot water heater. The pilot light recently went out. After lighting the pilot light, per the instructions, upon moving the botton to the ''on'' position the gas comes on as if there was a demand for hot water. Upon trying the hot water in the bath, the water was extremely hot. When tried in the kitchen the water would no longer heat. Returning to the heating unit, it was found that the pilot was again off. I assume that the unit over heated and the gas was automatically shut off. When relighting the pilot a second time, the same result occured. My question is why, when the button is moved to the on position, the gas comes on or is this normal?

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  • Anonymous Apr 12, 2009

    I believe the pilot thermocouple is bad -
    I have similar problem - instead of small pilot light there is a large flame. water heater responds to water demand but the "pilot flame' is far too large.

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Please someone help....i have the 125bng..it will not stay lit...i have changed the thermocoupler and the gas valve...spending alot of money but no result..

Posted on Sep 10, 2012

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It is most likely the water valve stuck open, telling the controls to heat water, that it thinks is flowing through it, but without actual flow it overheats. Depending on how comfortable you are with taking it apart yourself, you could open the valve to see if it just needs cleaned out, or if you have a ruptured diaphragm or some other cause.

Posted on Apr 22, 2009

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